Default minHeight is 200. You can change it by adding hs.minHeight to your Highslide settings between the head tags or in your highslide.config.js file (affects all popup in your page - hs.minHeight = 500;), or inline to the onclick (affects one specific popup - onclick="return hs.htmlExpand(this, { objectType: 'iframe', minHeight: 500 })" ).
Hilde
Highslide Support Team
Overview of my Highslide sample pages: RoadRash.no